README: Document formats and Git repository

Count the formats using:

    grep '^".*",$' src/datetime-formats.c | wc -l

List the date formats using:

    sed -n 's|^/\* \(.*\) \*/$|\1|p' src/datetime-formats.c

(But ignore 'Time only', 'ISO 8601: 1969-12-31 delimited by "T"',
'ISO 8601: 19691231 delimited by "T"', and '19691231'.)

Math to 704:

    43 (date formats) * 2 (positions: before or after time) = 86
    86 + 1 (time-only no-date format) = 87
    87 * 8 (time formats) = 696
    696 + 8 (special formats) = 704

1 files changed, 89 insertions, 0 deletions

diff --git a/README b/README
index c953475..a9b0a9a 100644
--- a/README
+++ b/README
@@ -6,6 +6,95 @@ commands, e.g.:
 
     @ 18:00; printf '\a'
 
+Date/Time Formats
+-----------------
+
+@ currently supports 704 date/time formats, any one of which may be
+specified on the command line in one or more arguments.  All spaces
+shown below are optional.
+
+A time may be specified in any of the following eight formats:
+
+  * 7:00:01 PM
+  * 19:00:01
+  * 7:00 PM
+  * 19:00
+  * 70001 PM
+  * 190001
+  * 700 PM
+  * 1900
+
+A date may be omitted or specified (before or after the time) in any of
+the following 43 formats:
+
+  * Wed
+  * 1969-12-31
+  * 1969/12/31
+  * 12-31-1969
+  * 12-31
+  * 12/31/1969
+  * 12/31
+  * 31 Dec 1969
+  * 31 Dec
+  * 31-Dec-1969
+  * 31-Dec
+  * 31/Dec/1969
+  * 31/Dec
+  * Wed 31 Dec 1969
+  * Wed 31 Dec
+  * Wed 31-Dec-1969
+  * Wed 31-Dec
+  * Wed 31/Dec/1969
+  * Wed 31/Dec
+  * Wed, 31 Dec 1969
+  * Wed, 31 Dec
+  * Wed, 31-Dec-1969
+  * Wed, 31-Dec
+  * Wed, 31/Dec/1969
+  * Wed, 31/Dec
+  * Dec 31, 1969
+  * Dec 31
+  * Dec-31-1969
+  * Dec-31
+  * Dec/31/1969
+  * Dec/31
+  * Wed Dec 31, 1969
+  * Wed Dec 31
+  * Wed Dec-31-1969
+  * Wed Dec-31
+  * Wed Dec/31/1969
+  * Wed Dec/31
+  * Wed, Dec 31, 1969
+  * Wed, Dec 31
+  * Wed, Dec-31-1969
+  * Wed, Dec-31
+  * Wed, Dec/31/1969
+  * Wed, Dec/31
+
+Additionally, the following eight date/time formats are supported:
+
+  * 1969-12-31T19:00:01
+  * 1969-12-31T19:00
+  * 19691231T190001
+  * 19691231T1900
+  * 19691231 70001 PM
+  * 19691231 190001
+  * 19691231 700 PM
+  * 19691231 1900
+
+Development
+-----------
+
+@ is maintained in a Git repository.  Browse:
+
+    http://git.pehjota.net/atsign/atsign.git/
+
+Clone:
+
+    git://git.pehjota.net/atsign/atsign.git
+
+Patches may be e-mailed to the author, P. J. McDermott <pj@pehjota.net>.
+
 Copyright
 ---------